(a) Field of the Invention
The present invention relates to a variable focal length lens system and, more particularly, to a variable focal length lens system utilizing fewer numbers of lenses.
(b) Description of the Prior Art
Formerly a variable focal length lens system with a comparatively large vari-focal ratio has been widely used especially for a video-camera lens. It generally comprises first lens unit having a positive refractive power, second lens unit having a negative refractive power and a function to vary focal length of the lens system, third lens unit having either positive or negative refractive power and a function to keep a certain distance from the image surface at the time of changing a focal length, and fourth lens unit having a positive refractive power and a capacity of image formation.
Moreover, as a variable focal length lens system with a comparatively small vari-focal ratio, it comprises first lens unit having a negative refractive power, and second lens unit having a positive refractive power, and, as is well known, a lens system which changes a focal length by moving an airspace between above two lens units. This variable focal length lens system has a function of image formation in the second lens unit.
In these known vari-focal lens systems, the lens unit having the function of image formation which is located on the most image side among lens units (hereinafter, the lens unit is referred to as an "imaging lens unit") contains the largest number of lenses among these vari-focal lens systems.
Besides, as the refractive power of each lens of the imaging lens unit is strong, it is time-consuming for processing lenses and lens burrels and assembling them, and therefore the manufacturing cost of the imaging lens unit will be high.
In order to reduce the number of lenses of this imaging lens unit, a spherical homogeneous lens has its own limit in case composing of this lens system. For this reason, an aspherical lens is utilized or a graded refractive index lens (GRIN lens) is hitherto known to be used.
A radial GRIN lens, whose refractive index is varied from the optical axis to the environs of the lens, i.e. in the direction of radius, has an especially good correction of aberrations. Therefore, it can be possible to reduce the number of lenses, to make a compact lens system and to minimize in cost if it can be well exploited.
Lens system disclosed, for example, in Japanese published unexamined patent applications Nos. 61-249015 and 61-249016 are known as the above-mentioned variable focal length lens system, particularly imaging lens unit whose number of lenses is reduced by using GRIN lenses.
Among these former vari-focal lens systems, the former comprises two lens units, one negative and the other positive, and a positive imaging lens unit applies a radial GRIN lens to reduce a number of lenses. The latter is arranged so as to locate an imaging lens unit close to the image side by utilizing a radial GRIN lens to reduce the number of lenses.